Locality Groups Overview

Shared memory multiprocessor computers contain multiple CPUs. Each CPU can access all of
the memory in the machine. In some shared memory multiprocessors, the memory architecture
enables each CPU to access some areas of memory more quickly than other
areas.

When a machine with such a memory architecture runs the Oracle Solaris
software, providing information to the kernel about the shortest access times between a given
CPU and a given area of memory can improve the system's performance. The
locality group (lgroup) abstraction has been introduced to handle this information. The lgroup
abstraction is part of the Memory Placement Optimization (MPO) feature.

An lgroup is a set of CPU–like and memory–like devices in which each
CPU in the set can access any memory in that set within
a bounded latency interval. The value of the latency interval represents the least
common latency between all the CPUs and all the memory in that lgroup.
The latency bound that defines an lgroup does not restrict the maximum latency
between members of that lgroup. The value of the latency bound is the
shortest latency that is common to all possible CPU-memory pairs in the group.

Lgroups are hierarchical. The lgroup hierarchy is a Directed Acyclic Graph (DAG) and
is similar to a tree, except that an lgroup might have more than
one parent. The root lgroup contains all the resources in the system and
can include child lgroups. Furthermore, the root lgroup can be characterized as having
the highest latency value of all the lgroups in the system. All of
its child lgroups will have lower latency values. The lgroups closer to the
root have a higher latency while lgroups closer to leaves have lower
latency.

A computer in which all the CPUs can access all the memory
in the same amount of time can be represented with a single lgroup
(see Figure 5-1). A computer in which some of the CPUs can access some
areas of memory in a shorter time than other areas can be represented
by using multiple lgroups (see Figure 5-2).

Figure 5-1 Single Locality Group Schematic

Figure 5-2 Multiple Locality Groups Schematic

The organization of the lgroup hierarchy simplifies the task of finding the nearest
resources in the system. Each thread is assigned a home lgroup upon creation.
The operating system attempts to allocate resources for the thread from the thread's
home lgroup by default. For example, the Oracle Solaris kernel attempts to schedule
a thread to run on the CPUs in the thread's home lgroup and
allocate the thread's memory in the thread's home lgroup by default. If the
desired resources are not available from the thread's home lgroup, the kernel can
traverse the lgroup hierarchy to find the next nearest resources from parents of
the home lgroup. If the desired resources are not available in the home
lgroup's parents, the kernel continues to traverse the lgroup hierarchy to the successive
ancestor lgroups of the home lgroup. The root lgroup is the ultimate ancestor
of all other lgroups in a machine and contains all of the machine's
resources.

The lgroup APIs export the lgroup abstraction for applications to use for observability
and performance tuning. A new library, called liblgrp, contains the new APIs. Applications
can use the APIs to perform the following tasks: